This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. PURKINJE CELL DENDRITIC SPINE DENSITY FROM CORRELATED LIGHT AND ELECTRON MICROSCOPY This project is an international collaboration between Dr. E. De Schutter (OIST, Japan) and Dr. F. Santamaria (Department of Biology, University of Texas at San Antonio, USA, fidel.santamaria@utsa.edu) It was previously shown that spines are an important cause of anomalous diffusion in Purkinje cell dendrites, due to trapping of molecules in dendritic spine heads (Santamaria et al. 2006). More recently we repeated this study, comparing Purkinje and CA1 pyramidal cells. We combined flash photolysis of caged Rhodamine dextran and two-photon fluorescent microscopy to track changes in concentration gradients along the dendrites of both cell types. We confirmed our previous findings and could show in pyramidal cells that the degree of anomalous diffusion depended upon spine density. However, there is no consensus on the spine density of Purkinje cells in the literature. We wish to use serial tomography to determine an accurate measure of spine density in rat Purkinje cells and to compare the values generated using diffraction-limited light microscopy.